1. Field Of The Invention
The present invention relates to a method and apparatus for testing a subsurface formation for fluid retention potential and more particularly to such a method and apparatus which are particularly well suited to the location and evaluation of porous subsurface formations having impervious overlaying structures forming downwardly facing pockets capable of capturing and retaining natural gas supplied to the porous formation until needed.
2. Description Of The Prior Art
The prior art is typified by such U.S. patents as the Allen U.S. Pat. No. 3,062,012; the Palmer U.S. Pat. No. 3,114,256; the Prats U.S. Pat. No. 3,604,256; the Kuo et al U.S. Pat. No. 3,636,762; the Chase, Jr. et al. U.S. Pat. No. 3,690,167; and the Prats U.S. Pat. No. 3,711,360. These patents relate primarily to methods and apparatus for studying, for various purposes, the structure and characteristics of rock and soil formations.
Applicant Sayer's U.S. Pat. No. 3,805,587 and currently pending U.S. patent application, Ser. No. 507,275, filed Sept. 19, 1974, and applicants Sayer and Wright's currently pending U.S. patent application, Ser. No. 422,319, filed Dec. 3, 1973, relate to various methods and apparatus useful in the location and evaluation of geothermal sources of energy.
It is known in the prior art to store natural gas in porous subsurface formations captured by interfacing overlaying impervious structures. Such natural gas may be that derived from oil wells and returned to the earth for later recovery and use. It is also known to store natural gas in subsurface formations adjacent to industrial and metropolitan areas for later recovery during periods of need such as during the winter months. Such subsurface storage has a myriad of advantages over the surface storage of natural gas. Where a suitable formation is located, elaborate and expensive surface storage facilities need not be constructed and maintained. The hazards of explosion and fire are considerably reduced. Once a suitable formation is located, it can be used repeatedly and virtually indefinitely with very little maintenance and in many cases with a storage capacity of virtually unlimited proportions for all practical purposes.
However, the primary difficulty with such subsurface storage of natural gas is the difficulty in locating formations of suitable structure. The porosity and permeability of the formation may not be sufficient to accommodate the injection and extraction of natural gas. The size of the porous formation may not be sufficient for storage purposes. The overlaying interface may, itself, be so porous as to permit natural gas to bleed off over a period of time. Similarly, the subsurface formation may allow the natural gas to leak into sewer systems, basements and the like so as to constitute an explosion and fire hazard. The interface may not be sufficiently vaulted, or in other words, may not have downwardly facing pockets of sufficient capacity to entrap the natural gas for subsequent extraction. There may also be subsurface bodies of water or steam which may carry away or dissipate the natural gas. Similarly, there may be substances in the formation or adjacent to the formation which contaminate the natural gas so as to make it unfit for later use.
For all of these reasons, it is necessary to test a likely formation for its potential to retain natural gas prior to injecting substantial quantities of the natural gas into the formation. Conventionally, the only method of which the applicants are aware for accomplishing this operation is to drill a borehole into the formation; pump a given quantity of natural gas into the formation; drill a plurality of boreholes laterally of the main borehole into the formation; and test the formation through the plurality of lateral boreholes in an attempt to determine the location, dissipation and size of the quantity of natural gas.
This method is of little practical value. Because of the extreme expense involved in drilling boreholes, particularly where great depths are involved, such conventional approaches have been practical only in a very few instances. Even where the method is employed, the test results are of necessity superficial due to the impracticality of drilling the considerable number of boreholes which is required to obtain a clear indication of the position, size and concentration of the natural gas within the formation. Furthermore, the existence of the plurality of boreholes themselves allows the natural gas to escape from the formation thereby distorting any test results obtained. Thus, in the vast majority of instances, the possibility of such subsurface storage is simply not pursued. This frequently results, in the case of the drilling of oil wells, in the illegal release of the unwanted natural gas into the atmosphere or the discarding of the gas by burning.
Therefore, it has long been known that it would be desirable to have a method and apparatus for testing subsurface formations for fluid retention potential which can be employed to determine the characteristics of the subsurface formation and its suitability to the storage of fluids such as natural gas over prolonged periods of time for recovery when required and to have such a method and apparatus which can be employed at only a small fraction of the expense of conventional methods and apparatus with a reliability and efficiency fully acceptable to adoption as routine practice.